Background The immature fetal lung development, especially immature typelⅡalveolar epithelial cells (AECⅡ), is an important pathogen of lung development diseases including bronchopulmonary dysplasia (BPD), respiratory distress syndrome(RDS) in premature infants.MicroRNAs (miRNA) as a class of vital regulatory molecules regulate most of all the biological processes.Recently, our team has performed miRNA profiling at 3 time points (E16, E19, E21) of the developing fetal rat lung and newly found miR-146b continuously substantially increase from E16 to E21.So we speculate that miR-146b may have influences on physiology of AECⅡ.Objective The aim of this study was to clarify whether miR-146b influences the physiology of AECⅡ and the possible mechanism.Methods we isolated type Ⅱalveolar epithelial cells from 19 days fetal rat lung by trypsin and collagenase and miR-146b was overexpressed with adenovirus vector in those cells.Then we detected the proliferation, transdifferentiation, surfactant proteins (SPs) and EGFR expression in primary AECⅡ using MTT assay, immunofluorescence, real-time PCR, western blot respectively.Results In this study, we successfully isolated AECⅡ from 19 days fetal rat lung.Above all, we demonstrated that overexpression miR-146b in AECⅡ is capable to inhibit proliferation of AECⅡ and mRNA expression of SP-B, but had no effect on the SP-C expression and transdifferentiation from AEC Ⅱ to AECⅠ.Furthermore, we identified that EGFR may involve in miR-146b regulation on AECⅡ.Conclusion These results suggested that miR-146b negatively regulated AECⅡproliferation and partially inhibited SPs expression in primary AECⅡ and EGFR may involve in that process.